Not applicable.
This invention relates to fluid power systems, specifically to a device for the determination of the level of solid particulate contaminants within the working fluid in a fluid power system or lubrication system.
Although fluid power systems and lubrication systems generally have the reputation of high reliability, the cost of a system failure can be significant, through damage, down-time and, in extremis, danger to personnel. There is overwhelming evidence that associates a fluid power system""s reliability to the level of particulate contamination present in the working fluid. It has been establish,ed that clean systems suffer fewer problems with component wear, seal breakdown awl, most important, with catastrophic failures such as valve seizure. It follows that, by determining the particulate contamination level of a fluid system, component and system health can be monitored. Fluid contamination monitoring not only establishes when fluid and filters should be cleaned or replaced but, also, forecasts impending component failurexe2x80x94thereby allowing predictive maintenance procedures to be initiated.
One of the most common techniques for assessing fluid continuation is the Patch Test. A small sample of working fluid, previously drained from the fluid system, is forked through a filter membrane (patch). The degree of contamination is indicated by the discoloration of the patch. The problem with this procedure is that the small fluid sample is unlikely to represent overall system contamination level. Additionally, the test cannot readily distinguish particulate sizes and quantitative measurements of contamination levels (in terms of ISO, NAS, or ASTM numbers) are impossible. Finally, the procedure typically takes two hours or more and must be carried out under laboratory conditions to avoid extraneous contaminants from entering the sample during measurement.
Another common technique for contamination monitoring is based on optical counter technology. Here a beam of light (white or laser) is directed through a sample of the working fluid whereupon it impinges on a photo-detector arrangement that senses both the size and occurrence (concentration) of the particulates. Particulate counters are available as static, offline monitors (requiring a sample of fluid to be tested under laboratory conditions) or as portable on-line monitors (temporarily integrated with the fluid power system). Optical counter technology has several problems: Firstly, at high concentrations of particles, monitor response becomes saturated. Secondly, sensor collaboration is set against a standard contaminant such as AC dust or glass spherical particles; however, wear debris is rarely spherical, giving rise to spurious results. Furthermore, optical counters cannot distinguish between water and solid particlates and may report that a system is contaminated when it is not. Entrained air and translucent particles, such as quartz and glass, also cause significant problems. Finally, optical counters are delicate in construction, expensive, and are highly sensitive to vibration and pressure ripple which affect the optical light-patch and may cause anomalous readings.
A further approach utilizes sensors to measure he pressure differential across a filter medium. The level of fluid contamination is estimated by the time taken to reach a pre-determined differential as filter blockage takes place. It is a problem with this method that it is highly sensitive to changes in pressure, flow rate, viscosity and temperature in the fluid upstream from the monitor. Accordingly, some inventors have included a second reference filter assembly to provide a computer compensated output, as in U.S. Pat. No. 4,685,066 to Hafele et al (1987). This adds significantly to monitor expense and complexity. Also, filters cannot be flushed adequately for continued operation. Therefore, filters must be changed following each measurement cycle thereby confining their adaptability and increasing running costs.
To redress some of these deficiencies, U.S. Pat. No. 3,050,987 to Osgood et al (1962) assesses contamination using an on-line monitor hat passes the working fluid through a narrow orifice (gap) between two flat displaceable surfaces. When the gap blocks with particulates, one surface is forced to move in relation to the other and the nature of the resulting movement is used to assess the degree of continuation. It is an advantage of the blockage technique in that it gives a direct indication of the contamination that is likely to cause problems in fluid power systems because the sensor gap is sized to be representative of the most critical gaps in the system. However, although Osgood""s method indicates the presence of contaminants, it has the problem characterized by poor repeatability because the relative movement between two blocked plates is also highly dependent upon oil viscosity, temperature, pressure fluctuations, and contaminant shape and substance The method is, therefore, highly unpredictable in nature and unlikely to give quantitative measurement of contamination level. Due possibly to these reasons and others, Osgood""s method has not been commercially developed and, to our knowledge, has never been available as a marketable product.
U.S. Pat. No. 4,468,954 to Lanctot et al (1984), and U.S. Pat. No. 4,495,799 to Fisher et al (1985) again assesses contamination using the xe2x80x9cblockage techniquexe2x80x9d. The method is characterized by passing the working fluid through a narrow annular gap and using the pressure differential across the gap to trigger a counter at a pre-determined contamination level. The cycling rate of the counter is used as an indication of the particulate concentration. It is a problem with the method that pressure within a fluid power system is rarely constant and working pressure changes, machinery vibration, as well as ripple effect, would cause the counter to operate, thereby producing false readings. Working pressure changes would also cause relative movement between the two elements of the gap, thereby releasing trapped particles prematurely and causing erroneous results. Correction of the results produced by this method to compensate for such effects would require substantial additions in both complexity and cost. Due possibly to these reasons and others, Lanctot""s method has not been commercially developed and, to our knowledge, has never been available as a marketable product.
U.S. Pat. No. 4,599,893 (1986), continued by U.S. Pat. No. 4,663,966, (1987) both to Fisher et al, also employ the blockage technique. These patents are characterized by the method of introducing a sample of working fluid of pre-determined volume into a cylinder (typically 1 liter) and forcing a piston, accommodating the annular gap, from one end of the cylinder towards the other. When the gap blocks, the progress of the piston is halted. The distance traveled by the piston may be used to assess fluid contamination level, This method has several problems: In order to measure at clean concentration levels (NAS 3/ASM 0), the monitor must be typically over one meter in length and weigh as much as 80 lbs., thereby severely limiting its portability and application. Furthermore, its measurement cycle time is typically over one hour, which also limits its use as a portable device. Also, the means used to communicate force to the piston to move it through the fluid is prone to flexing, tending to distort the gap thereby causing poor repeatability of results. Finally, the distance traveled by the piston is not only related to the level of contamination but, also, to the degree of force used to move the piston. This force may not be constant or repeatable over time. Due possibly to these reasons and others, Fisher""s method has not been developed commercially and, to our knowledge, has never been available as a marketable product.
U.S. Pat. No. 5,095,740 to Hodgson et al (Mar 1992) is a simplification of Fisher""s design (U.S. Pat. No. 4,663,966). Working fluid is passed through a filter membrane al system pressure and the volume collected therethrough. Volume to block is then read directly. Hodgson""s method is characterized by poor repeatability. Filter membranes are composed of random material providing an extremely random range of cavities for trapping debris. Also, the volume of the collected fluid used to back-flush the filter is inversely proportional to the contamination level. Therefore, should the filter block quickly with highly contaminated fluid, the flushing volume available is small and unlikely to flush the membrane adequately in preparation for the next cycle; furthermore, back-flushing a filter is known not to be fully effective. Finally, unless the filters are replaced often (thereby reducing repeatability), such a method is also highly prone to membrane failure. Should the filter burst, or become permanently blocked, full system feed pressure is routed directly to the return side of the fluid power system having significant effects upon the system.
In accordance with the current invention, a device for determining the level of particulate contamination in a fluid power system comprising an inlet and outlet for working fluid, a silting device to collect contaminants, and a flow rate detector adjacent to the silting device.
Our invention employs the blockage technique and flow rate disk discrimination. Several objects and advantages of the patent invention are to provide a contaminant monitor that:
a) operates on-line as an integral part of the fluid system and in capable of continuous and uninterrupted operation;
b) provides repeatable measurements over a wide range of contamination levels and has a rapid operating cycle time since fall blockage is not necessary;
c) is compact (typically less than 20 cms.), easily portable and sufficiently rugged for field use;
d) is light (typically less than 15 lbs.) and compatible to a wide range of operational uses;
e) gives a direct indication of the solid particulate contamination most likely to cause problems (regardless of shape or substance) in a given fluid power system;
f) is unaffected by entrained air, water, vibration, and ripple;
g) is unaffected by working fluid changes or variations in fluid flow rate, viscosity or temperature;
h) is not prone to sensor gap distortions or premature release ol collected particles;
i) the flushing volume is independent to the level of contamination, thereby allowing full flushing each cycle.
Further objects and advantages are to provide a monitor that is simple and safe to use and inexpensive to manufacture; that is integrated on-line either as a permanent installation on a fluid power system or, in a form that is fully portable (hand-held); that warns when a pre-determined (but variable) contamination level is reached; that may be used in conjunction with and controlled by, a portable computer; that is sufficiently small and of low weight so as to be applicable to airborne (aircraft) use. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.